The present invention relates generally to a method of geophysical exploration and more particularly to a method for enhancing seismic data by attenuating undesired coherent noise in the seismic data.
In seismic prospecting, it is conventional to place a plurality of seismic receivers along the earth's surface at spaced locations. A plurality of seismic sources disposed at spaced locations along the earth's surface can then be activated to generate seismic waves which propagate outwardly in all directions. Vibrating devices, explosive devices, and impulsive devices are all exemplary of such seismic sources. Seismic waves thus generated are reflected, refracted, and diffracted from subsurface formation interfaces, and some of these diverted seismic waves are detected by the seismic receivers and can be processed to form a seismic signal. Such seismic signals can be displayed as seismic sections which contain information about the time, duration and intensity of the diverted seismic waves. Seismic sections can be studied to extrapolate information regarding the type and location of subsurface formations producing the diverted seismic waves. This information can, in turn, be employed to evaluate subsurface formations for oil- and gas-bearing potential.
Seismic energy which has generally been reflected only once from a reflecting subsurface interface is commonly referred to as a primary reflection event, whereas seismic energy which has been reflected more than once from a reflecting subsurface interface is commonly referred to as a multiple reflection event. Such reverberating seismic energy can produce multiple or secondary reflection events in the seismic data from one or more reflecting interfaces in the earth. Consequently, the presence of multiple reflection events or, more simply, multiples in the seismic data can result in confusing and oftentimes noninterpretable seismic data.
One method for attenuating multiple refraction events in the seismic data is to sort the seismic data into common depth point (CDP) gathers of seismic signals, normal moveout (NMO) correct the seismic signals of the CDP gather, and then dip filter the NMO corrected seismic data as described by Ruehle in U.S. Pat. No. 4,209,854. Such technique, however, is dependent upon differences in normal moveout velocities between the multiples and primary reflection events. Moreover, only a single choice of velocity per reflection event in the NMO moveout correction is permitted for each gather of seismic signals or for each iteration of the NMO correction.
More recently, Yanchak described in U.S. Pat. No. 4,907,205 a method for attenuating multiples by first sorting the seismic data into common end point (CEP) gathers of seismic signals, aligning selected multiples employing a two parameter moveout correction (i.e., moveout velocity and dip angle) and then spatially filtering the seismic signals to attenuate the aligned multiples. The effectiveness of such approach is not limited by the requirement that moveout velocities for primary and multiple be different; however, such technique nonetheless limits the choice of moveout velocity and dip angle to a single choice per reflection event for each iteration. Consequently, a need exists for an efficient method for removing coherent noise events in general, and multiple reflection events, in particular, which have any combination of spatially or temporally varying moveout velocities or dip angles.